Method of controlling the formation of crystals in molten metal as it solidifies



1957 D. J. CARNEY ET AL 7 METHOD OF CONTROLLING THEFQRMATION OF CRYSTALS IN MOLTEN METAL AS IT SOLIDIFIES 3 Sheets-Sheet 1 Filed May 21, 1952 HEAT NO 8X6303 INGOT 11 NO ADDITlON GRADE- 17 CR SECTION I SECTION 2 MIDDLE TRANSVERSE CUT BOTTOM YRANsvERsE CUT DENNIS J. CARNEY a BERNARD R. QUENEAU BYM INVENTOR5, J

1957 CARNEY ET AL 2,778,079

D. J. METHOD OF CONTROLLING THE FORMATION OF CRYSTALS IN MOLTEN METAL AS IT SOLIDIFIES Filed May 21, 1952 3 Sheets-Sheet 2 HEAT NO axeaoa INGOT 12 FECR ADDED 5E HON GRADE-l7 CH TRANSVER s: CUT

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DENNIS J. CARNEY 8r BERNARD R. QUENEAU INVENTOR5. MM?

Jan. 22, 1957 D. J. CARNEY ET AL 2,778,079

METHOD OF CONTROLLING THE FORMATION OF CRYSTALS I5 Sheets-Sheet 3 IN MOLTEN METAL AS IT SOLIDIFIES Filed May 21, 1952 DENNIS J, BERNARD R. QUEN INVENTOR5 MW United States Patent METHOD OF CONTROLLING THE FORMATION OF CRYSTALS 1N MOLTEN METAL AS IT SOLIDIFIES Dennis J. Carney, Chicago, 111., and Bernard R. Queneau, Pittsburgh, Pa., assignors to United States Steel Corpo ration, a corporation of New Jersey Application May 21, 1952, Serial No. 289,076

2 Claims. (Cl. 22215) This invention relates to a method for improving the quality of metal ingots by controlling the crystal formation on freezing thereof.

It is well recognized that the crystal formation in cooling ingots resulting from natural causes is not always favorable for subsequent Working by rolling. The normal dendri-tic crystal structure, resulting from the slow cooling of large ingots, if it persists to an exaggerated degree, leads to cracking during rolling unless special precautions are observed (The Making, Shaping and Treating of Steel, 6th ed., page 575). It is accordingly the object of our invention to predetermine the crystal formation instead of letting it proceed unregulated in a pattern fixed by conditions normally existing in an ingot mold.

Our invention contemplates the provision of a multiplicity of nuclei in the molten metal, each of which favors the formation of a crystal around it, The nuclei are afforded by seeding the bath with solid metal particles of definite characteristics in'relation to those of the metal being cast. More particularly, the metal particles used for seeding must have a space lattice similar to that of the metal of the bath at its solidification temperature'or a crystallographic plane in common therewith, and'substantially the same space-lattice pa rameter.

A complete understanding of our invention may be obtained from the following detailed explanation of the preferred practice and the specific examples presented therein, the results of some of which are shown in the accompanying illustrations in which,

Figure 1 is an etched longitudinal section through successive portions of a 17% chrome-steel ingot resulting from teeming and cooling in the known manner;

Figure 2 be series of cross-sections therethrough at the planes indicated (Figures 1 and 2 are included onl for contrast with Figures 3 and 4);

Figures 3 and 4 are sections similar to Figures 1 and 2 showing the effect on grain structure of a similar ingot produced by our invention;

Figure 5 is an etched cross-section through a bronze ingot which crystallized normally as a result of natural cooling;

Figure 6 is a similar view of an ingot of the same bronze treated according to our invention;

Figure 7 is a photomicrograph showing the crystallization of a Babbitt ingot on natural cooling; and

Figure 8 is a corresponding view of a similar ingot treated by our invention.

Generally stated, our invention consists in introducing in-to molten metal, a metal or alloy in finely divided form, such as power, which will have the effect of a nucleating agent, i. e., produce a multiplicity of points in the bath favorable to crystal formation. The powdered metal may be added to the molten metal just before or during the teeming thereof from a ladle into ingot molds. The added metal must have certain characteristics, viz;

(a) It mus-t have a space-lattice structure similar to that of the metal being treated, at its point of solidification; or

(b) A space-lattice parameter approaching that of the metal being treated, when in the liquid state, and a crystallographic plane in common therewith.

The space-lattice parameter is the length of an edge of the cube of the space lattice of atoms by the addition of which crystal growth occurs.

When comminuted metal having the required characteristics is added to the molten metal to be treated and the latter is teemed into ingot molds, the added finely divided metal, even though partially or substantially melted by the heat of the bath, creates a multiplicity of nuclei favoring the initiation of crystal growth. As a result, the freezing of the molten mass is accompanied by the formation of a greater number of crystals of smaller size than when the molten metal is permitted to solidify without the addition of solid metal. In other words, the particles of added metal seed the molten mass and increase the number of crystals forming therein on cooling. The ize of the crystals is thus reduced and the difliculty caused heretofore by crystals of excessive size is avoided.

For steel crystallizing in a body-centered cubic (B. C. C.) system, such as ordinary carbon steel, the powdered metal added thereto may be chromium, iron or molybdenum or alloys thereof. Metals forming a face-centered cubic (F. C. C.) system, such as 18-8 stainless steels, may be treated in accordance with'the invention by the addition of powdered nickel, iron, cobalt, or manganese and the austenitic alloys thereof. In the case of nonferrcus alloys such as bronze containing copper and 10% tin, or Babbitt containing 90% tin, 7% antimony and 3% copper, the desired grain refinement in ingots on freezing thereof has been obtained by adding powdered copper to bronze or tin to Babbitt.

The following tables show the positive results obtained in typical exam-pies of the invention, and also, for contrast, the results obtained without practicing the invention or using addition agents not having the properties required. In all the tables, the lattice parameter is that measured at room temperature and the size of equi-axed grains is that determined by observation without magnification at' the midpoint of the longitudinal axis, by reference to the A. S. T. M. grain-size chart for a magnification of 100. The size of the particles is indicated by the mesh of the screen which they will pass through (minus figures) and the mesh of the screen on which they will remain (positive figures).

The data given in all the tables were obtained from heats, weighing approximately 27 pounds, which were poured into 3 diameter graphite molds with hot tops.

equi=axed grains. In the case of 27% chromium steel, different from that of 27% Cr steel) had no apparent heats poured below 2800 F. without adding powdered efiect on the solidification pattern.

TABLE I 27% Cr stainless steel-body centered cubic lattice parameter 2.87 A. units Addition Grain Refinement Pour Temp. Columnar Equiaxed Kind Quantity, Lattice, Structure Mesh F.) Grain Grainsize lb. per ton A. Units Length (ASIM) New 2,790 n 7-8 2 2.87 B. 0.0 40 to +60, 2.800 9 2 3.10 B. 0. 0 -e5 to +100-.. 12,335 0 9 2 3.52 110.0 2,800 as s a 2 o. P. H -100 2,795: at 7-8 metal were relatively fine-grained whereas heats poured In Table II are presented similar data for higher pourab'ove this temperature were coarse-grained. ing temperatures. 'Ferromolybdenum is a little more Table I gives the results obtained by an application of effective than ferrochromiutn at the higher pouring temour method to 27% Cr steel. These data show that peratures.

TABLE II 27% Cr stainless steel-body centered cubic lattice parameter 2.87 A. units Addition Grain Refinement Pour f Temp. Columnar Equiaxed Kind Quantity, Lattice, Structure Mesh F.) Grain Grainsize lb. per ton A. Units Length (ASTM) fin.)

None 2,840 1 5-6 .2 40 to 2, 860 s 2 -e5m +100... 2, 855 an 9 2 to +100.-. 2, 390 st 8 ferrochromium and ferromolybdenum powders (which The effect of the amount of powder added is shown have the same crystallographic structure and approxiin Table III. I At least two pounds per ingot of powder mately the same lattice dimension as the 27% Cr steel) was needed for appreciable grain refinement for this grade were very efiective in producing grain refinement as shown in this size of ingot.

TABLE III 27% Cr stainless steel-body centered cubic lattice param- 'eter 2.87 A. units Addition Grain Refinement V Pour i Y Temp. columnar, Equlexed Kin'd Quantity, Lattice, Structure Mesh -(-F.) Grain Grainsize lb. per ton A. Units Lzength (ASTM) .25 3.10 e5 to +100... 2,850 as 7-8 1.00 3.10 "65 to +100... 2,860 7-8 2.00 3.10 65 to +100... 2,855 M 9 4. 00 3. 10 -05 to +100.. 2,845 9 by the increase in grain-size number, while nickel and Data tor the 17% Cr grade of stainless steel is shown "alumina powders (which have a crystallographic structure in Table IV.

TABLE IV 17% Cr stainless steel-body centered cubic lattice;parameter 2.87 A. units I Addition Giuin'Retlne'inent Pour I Temp. Columnar Equlaxed Kind Quantity, Lattice, Structure Mesh F.) Grain Grainslze l'o.'per ton A. Units I Lallllgth (ASTM) None Y i 2, 815 V :1 8 2 3.10 B. C. G -35 to +05.-.. 2,315 at 9 from 1 ounce to four pounds per ton of the addition agent will suifice for producing grain refinement.

The use of our method results in control of the grain size of metals which improves the as cast structure and markedly reduces segregation and pipe in the ingots.

We claim: 1. In a method of refining the grain of ferritic steel TABLE V 18-8 stainless steel-face centered cubic lattice parameter 3.57 A. units Addition Grain Refinement Pour Temp Oolumnar Equlaxed Kind Quantity, Lattice, Structure Mesh (F.) Grain Gralnsize lb. per ton A. Units Lgngth (AS'IM) None 2, 800 1% None 2, 750 1% None 2, 705 1% None 2, 625 1% 3.52 F. O. C 60 2,835 2 3.57 F. O. 0... 2,805 V 2 3. 57 F. C. C.--" 35 2, 825 2 2.87 B. G. G -60 ,810 1% In addition to the above examples, two commercial ingots of 17% Cr stainless steel were cast into 16" x 16" x 72" big-end-up hot-top ingots. To one of the ingots, 2 pounds per ton of to +35 mesh ferrochromium powder was fed through a chute into the ingot mold while teeming. The two ingots were split and macro-etched. The results are shown in Figures 1 and 2. The ingot to which powder was added (Figure 2) has a finer grain and shows no evidence of intergranular shrinkage which is very evident in the natural ingot.

Our method has also been applied to non-ferrous metal systems. Figure 3 shows the results of adding 4 grams of copper powder to a three-pound casting of 90% Cu, 10% Sn. Aluminum may be added instead of copper. Figure 4 shows the results of adding 1.5 grams of tin powder to a one-pound tin Babbitt casting (90% Sn, 7% Sb, 3% Cu). Marked grain refinement was obtained by seeding in both of these cases.

One of the requirements of the process is that the addition agent be pulverized before introduction into the metal to be treated. The fineness is not too critical, however, and may vary from 8 mesh to 300 depending on the mold conditions, degree of superheat and the melting point of the metal.

In carrying out the invention, the metal to be treated is made in the usual manner, and the seeding alloy or metal is added in powder form to the liquid metal just before or during pouring. Our experience indicates that as cast, the steps including adding to the steel while molten, but not substantially earlier than the teeming thereof into a mold, from one ounce to four pounds per ton of a metal selected from the group consisting of chromium, iron, molybdenum, silicon and ferritic alloys thereof comminuted to from 8 to 300 mesh.

2. In a method of refining the grain of austenitic steel as cast, the steps including adding to the steel while molten, but not substantially earlier than the teeming thereof into a mold, from one ounce to four pounds per ton of a metal selected from the group consisting of nickel, iron, manganese, cobalt and austenitic alloys thereof comminuted to from 8 to 300 mesh.

References Cited in the file of this patent UNITED STATES PATENTS 963,973 Wright July 12, 1910 2,260,226 Kirkham Oct. 21, 1941 2,302,999 Obrien Nov. 24, 1942 2,519,593 Olfenhauer Aug. 22, 1950 FOREIGN PATENTS 212,725 Great Britain Mar. 20, 1924 145,088 Austria Mar. 25, 1936 OTHER REFERENCES The Iron Age, vol. 169, issue 7, page 140. Pub. Feb. 14, 1952. 

1. IN A METHOD OF REFINING THE GRAIN OF FERRITIC STEEL A CAST, THE STEPS INCLUDING ADDING TO THE STEEL WHILE MOLTEN, BUT NOT SUBSTANTIALLY EARLIER THAN THE TEEMING THEREOF INTO A MOLD, FROM ONE OUNCE TO FOUR POUNDS PER TON OF A METAL SELECTED FROM THE GROUP CONSISTING OF 